1. Field of the Invention
The invention disclosed herein relates to measuring strain and, in particular, to measuring the strain with an optical fiber.
2. Description of the Related Art
Hydrocarbons are generally recovered through boreholes penetrating reservoirs of the hydrocarbons. Various types of structures may be disposed in the boreholes for the recovery process. During well completion, one type of structure known as a casing or tubular is disposed in a borehole. The casing, similar to a pipe, is used to contain the hydrocarbons flowing to the surface of the earth for recovery.
Structures such as casings disposed in boreholes can be exposed to harsh environments. The harsh environments include high temperature, high pressure, and high stress. The high stress can result from the high temperature, the high pressure, high loads or high vibration. When exposed to the high stress, the structure can experience strain. Because of the high cost of well completion, it is important to monitor the strains experienced by the downhole structures to prevent damage. Thus, strain sensors may be attached to the structure at various points to monitor the strains.
One type of strain sensor uses an optical fiber to measure the strains experienced at various points along the optical fiber. Because the optical fiber is attached to the structure, the optical fiber will experience the same strain as the structure.
In general, a series of identical fiber Bragg gratings is etched into the optical fiber. Each fiber Bragg grating reflects light at a certain frequency depending on such factors as the magnitude of the refractive index changes in the optical fiber and the distance between the refractive index changes. Thus, as the optical fiber experiences the strains experienced by the structure, the distance between the refractive index changes causing the frequency of the reflected light to change. Measuring a change in the frequency of the reflected light can then be related to the strain experienced by the structure.
Thousands of fiber Bragg gratings can be etched into one optical fiber to measure strains at hundreds or thousands of locations on a structure. Unfortunately, by having many fiber Bragg gratings etched into one optical fiber, a point is reached when the total reflectivity of the optical fiber is about fifteen to twenty percent. When this point is reached, “ringing” may occur. Ringing is an optical phenomenon wherein light undergoes multiple reflections within the optical fiber. That is the fiber Bragg gratings begin to act as an optical cavity to continuously reflect light between the gratings. When ringing occurs, it is difficult to accurately measure the frequency of the reflected light from each grating and to determine the associated strain.
Therefore, what are needed are techniques to measure strains experienced by a structure downhole. Preferably, the techniques minimize the probability of ringing occurring.